12/12/2023 0 Comments Scala implicit![]() the relative weights are the same, and A takes no implicit parameters but B does, or.the relative weight of A over B is greater than the relative weight of B over A, or.The following paragraph in the SLS §6.26.3 is affected by this change:Īn alternative A is more specific than an alternative B if the relative weight of A over B is greater than the relative weight of B over A.Īn alternative A is more specific than an alternative B if If both alternatives take context parameters, we try to choose between them as if they were methods with regular parameters. All else being equal, an alternative that takes some context parameters is taken to be less specific than an alternative that takes none. The rule for picking a most specific alternative among a set of overloaded or implicit alternatives is refined to take context parameters into account. So the following code snippet would be ambiguous in Scala 3:ħ. Scala 2 gives a lower level of priority to implicit conversions with call-by-name parameters relative to implicit conversions with call-by-value parameters. By contrast, most (but not all) divergence errors in Scala 2 would terminate the implicit search as a whole.Ħ. This also makes sense: Encountering a divergent implicit means that we assume that no finite solution can be found on the corresponding path, but another path can still be tried. A divergent implicit is treated as a normal failure, after which alternatives are still tried. The treatment of divergence errors has also changed. For any query type Q, NotGiven succeeds if and only if the implicit search for Q fails.ĥ. But there is now a new special type which implements negation directly. With the new cleaned up behavior these techniques no longer work. Scala 2's somewhat puzzling behavior with respect to ambiguity has been exploited to implement the analogue of a "negated" search in implicit resolution, where a query Q1 fails if some other query Q2 succeeds and Q1 succeeds if Q2 fails. By contrast, Scala 2 would have rejected the search for A as ambiguous, and subsequently have classified the query b(implicitly) as a normal fail, which means that the alternative c would be chosen as solution! This makes sense, after all there are two possible solutions, b(a1) and b(a2), neither of which is better than the other and both of which are better than the third solution, c. ![]() This query would now be classified as ambiguous. If an ambiguity is encountered in some recursive step of an implicit search, the ambiguity is propagated to the caller.Įxample: Say you have the following definitions: ![]() The treatment of ambiguity errors has changed. If T is some other type, S includes the implicit scopes of all anchors of T.Ĥ.If T is a reference to an anchor of the form p.A then S also includes all term references on the path p.If T is a reference to an abstract type or match type alias named A, S includes a reference to an object A defined in the same scope as the type, if it exists, as well as the implicit scopes of T's given bounds.If T is a reference to an opaque type alias named A, S includes a reference to an object A defined in the same scope as the type, if it exists, as well as the implicit scope of T's underlying type or bounds.If T is a reference to an object, S includes T itself as well as the implicit scopes of all of T's parent classes.If T is a reference to a class, S includes a reference to the companion object of the class, if it exists, as well as the implicit scopes of all of T's parent classes.If T is some other type, the union of the anchors of each constituent type of T.ĭefinition: The implicit scope of a type T is the smallest set S of term references such that.If T is the this-type o.this of a static object o, the anchors of a term reference o.type to that object.If T is a singleton reference, the anchors of its underlying type, plus, if T is of the form (P#x).type, the anchors of P.If T is a reference to a type parameter, the union of the anchors of both of its bounds.If T is an alias of U, the anchors of U. ![]() If T is a reference to an anchor, T itself plus, if T is of the form P#A, the anchors of P.Opaque type aliases count as anchors only outside the scope where their alias is visible.ĭefinition: The anchors of a type T is a set of references defined as follows: References to packages and package objects are anchors only under -source:3.0-migration. In more detail, here are the rules for what constitutes the implicit scope of a type:ĭefinition: A reference is an anchor if it refers to an object, a class, a trait, an abstract type, an opaque type alias, or a match type alias. However, a reference to p.o.C outside of package p will have only b in its implicit search scope but not a. Both a and b are visible as implicits at the point of the definition of type C.
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